Producers of commercial electronic equipment, not to mention defense applications, are being forced to improve the performance of shielding devices to counteract electromagnetic interference complicated by the ever higher frequencies of energy generation in and beyond the megahertz range. Openings in cabinet enclosures for access doors, hinges, etc. must be so protected by the use of suitable shielding.
This has been an ongoing problem and various solutions have been proposed, these prior solutions however being unsatisfactory from one or more points of view. Thus, the prior devices have been insufficient from point of view of at least one or more of the following requirements:
(1) Attenuation of internal and external interferences using gasketing with resistive values with the range of 0.1 to 0.001 ohm-cm and selectable attenuation form 30 to 90 dB. PA1 (2) Low compression set of the gasket so long term mechanical sealing is maintained. PA1 (3) Superior physical properties of the gasket to prevent failure due to tearing forces. PA1 (4) Resistance to reduced electromagnetic shielding due to oxidation. PA1 (5) Ease of cleanliness maintenance. PA1 (6) Freedom from free metal chips as occurs with the initial fitting and subsequent deterioration of metallic meshes and metal assemblies. PA1 (7) A neat appearance of the shield. PA1 (8) Ease of installation and replacement. PA1 (9) Low cost relative to alternate designs. There are presently available four different types of cabinet shielding means which suffer one or more defects. These are as follows:
______________________________________ Product Principal Inadequacy ______________________________________ (1) Carbon Filled Elastomers Shielding ability falls off in the higher frequency range (2) Metal Filled Elastomers Poor physical properties (compression set, tear, etc.) (3) Metal Mesh with Hard to cut cleanly for Elastomeric Core corners; chips of free metal (4) Metallic Seals Difficult to keep clean, (Spring-like fingers) oxidation ______________________________________
In addition, it is to be noted that most metals are problematic in shielding against magnetic waves unless the metals are in extremely thick layers.